Electrolytic cell

ABSTRACT

A CELL FOR THE ELECTROLYSIS OF MOLTEN OXIDES, ESPECIALLY OF ALUMINA, IN WHICH THE ANODE IS SEPARATED FROM THE MELT BEING ELECTROLYSED BY A LAYER OF AN OXYGEN-ION-CONDUCTING MATERIAL, FOR EXAMPLE ZIRCONIUM OXIDE STABILISED WITH CALCIUM OXIDE OR OTERH OXIDES, WHICH IS RESISTANT TO THE MELT AT THE TEMPEARTURE OF THE ELECTROLYSIS.

Feb. 9, B, MARlNCEK ELECTROLYTIC CELL Filed May l5. 1967 IN ENTOR 9o/wrA/P//vcf/f mm MVSM United States Patent C 3,562,135 ELECTROLYTIC CELLBorut Marincek, Kusnacht, Switzerland, assignor to Swiss Aluminium Ltd.,Chippis, Switzerland, a joint-'stock company of Switzerland Filed May15, 1967, Ser. No. 638,249 Claims priority, application Switzerland, May17, 1966, 7 ,27 5 66 Int. Cl. B01k 3/04, 3/06; C22d 3/02 U.S. Cl.204-243 4 Claims ABSTRACT OF THE DISCLOSURE The electrolysis of moltenmaterials, for example of alumina, is carried out today with carbonanodes. In the case of alumina, the oxygen ions formed during theelectrolysis react with the carbon of the anode at the processtemperatures of 900 to l000 C. and form carbon dioxide, which is partlyreduced to carbon monoxide by the aluminum itself. Owing to theoxidation of the anode by the nascent oxygen, the carbon anode isconsumed and, in fact, if only carbon dioxide were to be formed, 334 kg.of carbon per ton of aluminum produced would be consumed. In practice,about 400 to 450 kg. of anode carbon are consumed per ton of aluminum,which corresponds to about 8 to 10% of the cost of crude aluminum. Ithas only recently been possible to reduce the consumption of anodecarbon even to this figure, and with the present method of Working usingcarbon anodes, reduction of the consumption of anode carbon below thetheoretical ly smallest amount, that is 334 kg. of carbon per ton ofaluminum, is not possible.

It has now been found that it is possible to carry out the meltelectrolysis of oxides without using carbon anodes, but using electrodeswhich are oxygen-resistant without necessarily being resistant to attackby the melt being electrolysed, so that the oxygen can be obtained as avaluable by-product of the process. About 600 cu. m. of pure gaseousoxygen are formed per ton of aluminum; the value of the oxygen is about3% of the cost of the crude aluminum. The oxygen which can be recoveredin carrying out the process according to the invention into effect canbe used for various oxidising processes, such as for example, steelproduction (by the oxygen blow method), the gasification of fuels (forproducing synthetic gas) and the preparation of reducing gases for ironreduction.

According to the invention we use an anode of any suitable conductingmaterial, and lwe separate this anode from the melt being electrolysedby a layer of material which is oxygen-ions-conducting but non-permeableto and resistant to the melt at the temperature of the electrolysis, sothat the oxygen ions diffuse through the layer and are then dischargedat the anode with the formation of oxygen gas. The anode itselfpreferably consists of an electron-conducting material which does notreact with oxygen or at least does not form with oxygen any compoundimpairing the conduction of electrons. Suitable materials includeheat-resistant alloys, platinum or other noble metals,electron-conducting oxides, such as for example, wustite, certainmaterials with semi-conductor properties, and metals with a passivatedsurface. The thickness of the oxygen-ion-conducting layer may be verysmall so that the voltage drop across it is also small; this reduceslosses of energy during the electrolysis.

I3,562,135 Patented Feb. 9, 1971 We have found that known stabilisedforms of zirconium oxide are very suitable as the material whichseparates the anode from the melt. By stabilised, we mean zirconiumoxide in which is incorporated proportions of other oxides such ascalcium oxide, magnesium oxide and yttrium oxide, whcih serve firstly tostabilise the cubic (fluorite) lattice of the zirconium oxide, andsecondly to confer on it the necessary oxygen-ion conductivity. Bysuitable choice of oxides and their proportions, a stabilised zirconiumoxide can have a resistance as low as l0 ohmscm. at l000 C. Otherrefractory oxides which have uorite lattices can be used, such as forexample, rare earth oxideuranium oxide compositions, thoriumoxide-uranium oxide compositions and cerium oxide suitably stabilisedwith calcium oxide or magnesium oxide. Substances which reduce thesolubility of the oxygen-ion-conductng material may be added to thefused melt.

The invention will be described hereinafter with specific reference tothe electrolysis of alumina for the production of aluminum. In such anelectrolysis, the oxygen ions which are formed in accordance with theequation diffuse through the oxygen-ion-conductive layer and aredischarged at the anode in accordance with the equation i.e. the oxygenions combine to form oxygen gas and electrons are released in theprocess. These electrons are conducted away by the anode. Other oxidessuch as for instance, MgO, NagO, CaO, Fe2O3, can also be electrolysed bythe process according to the invention and similar equations can beformulated. In the electrolysis of alumina for example, cells accordingto the invention afford the following advantages, inter alia, incomparison with the present state of the art.

(l) There is no consumption, or only a very low rate of consumption, ofanode material with the result that the rate of production of anodematerial can be substantially reduced.

(2) The formation of carbon scum in the bath results in a loss ofoperating efficiency, and this formation will not occur if the anodesare of material other than carbon.

(3) There is improvement of the quality of the aluminium metal produced,since no impurities, such as iron, silicon or vanadium are introducedfrom the anode material.

(4) There is less downtime of the cell, since the anodes do not have tobe replaced.

(5) There is a reduction of the consumption of uxing materials, sincethe cell can be sealed off more satisfactorily and this also gives animprovement in the shop atmosphere.

(6) Pure oxygen can be produced and collected as a byproduct.

(7) There is no re-oxidation of the liquid aluminum by carbon dioxideand thus, there is increased output from the cell and a reduction in theenergy required to produce a. given weight of aluminium.

Cells according to the invention can readily be adapted for automationof operation with for example, continuous addition of alumina to thefused melt and maintenance of constant interelectrode gap or cellvoltage.

Cells according to the invention may be constructed in two Ways. In thefirst of these, the anode is coated with or is in contact with the layerof oxygen-ion-conducting material over at least that part of its surfacewhich is immersed in the melt; the anode must then be in such a physicalstate that oxygen gas can pass through it.

The anode may be solid, in which case it must be porous, perforated orreticulated.

If the anode is solid, the layer of oxygen-ion-conducting material maybe applied directly to it by pressing or casting with subsequent dryingand sintering or by plasma spraying. Alternatively a body of thematerial may be preformed quite separately and put in Contact with theanode, if the latter is, for example, a metal network. As a furtherpossibility, a porous layer of platinum black may be applied to apreformed body of the material, and electrically connected to oneterminal of the current supply. This last proposal is found to be verysatisfactory, as platinum black is particularly suitable for thedischarge of oxygen ions and the formation and removal of oxygen gas.

The invention will now be described with reference to FIGS. l to 2 ofthe accompanying drawings, each of which represents a section through anelectrolytic cell for the electrolysis of fused alumina-cryolitemixtures. In each of the gures, a carbon tank 4 contains the fusedaluminacryolite melt indicated as 1, and the liquid, electrolyticallyproduced aluminum, which accumulates on the bottom of the cell and atthe same time acts as a cathode in the arrangements according to FIGS. land 2 is shown as 2. The fused melt is covered by a layer 3 consistingof solidified melt and alumina. A bus bar 5 conducts the current fromthe tank.

In FIGS. 1 and 2, the anode consists of a gas-permeable,electron-conducting body which is covered with the oxygen-ion conductivematerial over at least the portion of its surface immersed in the fusedmelt. In FIG. l, the oxygen-ion conductive material 8 is in the form ofa hollow cup-shaped body, the inner surface of which is lined with alayer 9 of platinum black as anode. The layer 9 is electricallyconnected to a terminal 7 which is itself connected to a source ofdirect current by means of a lead 6. During the electrolysis the oxygenions of the electrolyte diffuse through the oxygen-ion conductive layer8, are discharged at the surface of contact between the oxygen-ionconductive layer 8 and the layer of platinum black 9 and combine in thelayer of platinum black to form gaseous oxygen which collects in thehollow space 10 and escapes through a vent 11. In the constructionshown, the terminal 7 forms the upper end of the hollow space 10 andincorporates the gas vent 11. The electrons that are liberated flow offby way of the anode 9, the terminal 7 and the lead 6. The oxygen gasevolved can escape under atmospheric pressure, be drawn off underreduced pressure or be collected under pressure in excess of atmosphericin the space 10.

In FIG. 2, the oxygen-ion conductive material is in o metal oxidescontained in a molten electrolytic bath, this cell comprising acontainer for the melt being electrolysed, a cathode for contact withthe melt and a gas permeable anode resistant to the formation withoxygen of any compound impairing its conduction of electrons, a layer ofoxygen-ion-conducting material in direct electric contact at one sidewith said anode substantially over at least that part of its area to beimmersed in a melt in said container and freely exposed at its otherside to said melt, said layer being non-permeable to and resistant tothe melt at the temperature of the electrolysis, and a source of directcurrent connected between said anode and said cathode to maintain saidelectrolysis, said current during said electrolysis effecting thediffusion of oxygen ions through the layer and their discharge at theanode with the formation of oxygen gas which escapes through thegas-permeable anode which is uncovered on its other side.

2. A cell according to claim 1 in which the anode is in contact with thelayer of oxygen ion-conducting material over at least that part of thesurface which is immersed in the melt and is in such a physical statethat oxygen gas can pass through it.

3. A cell according to claim 2 in which the anode is solid, and isporous, perforated or reticulated.

4. A cell according to claim 3, in which the anode is formed by a porouslayer of platinum black applied to a preformed body of oxygen-ionconducting material.i

References Cited UNITED STATES PATENTS 3,141,835 7/1964 Rolin et al.204-67X 3,297,551 l/l967 Alcock 204-1 3,359,188 12/1967 Fischer 204-13,400,054 9/1968 Ruka et al. 204-l 3,403,090 9/1968 Tajiri et al. 204-1JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner U.S.Cl. X.R.

